How To Diagnose And Treat Plant Growth

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3 Table of Contents Introduction...1 Examining a field... 2 Tips for shipping to diagnostic laboratories Stage I: Scouting from emergence to knee-high... 4 Stage II: Scouting from knee-high to tasseling Stage III: Scouting from tasseling to maturity Disease symptoms Leaf diseases Smutting diseases Virus and virus-like diseases Fungal systemic diseases Stalk and root rot diseases Ear and kernel rots Insect injury symptoms and management recommendations Herbicide injury symptoms Herbicides that may be used or trigger symptomology in corn Herbicides listed by active ingredient and mode of action Nutrient deficiency symptoms Index... 54

4 Introduction This manual is prepared and distributed by Monsanto Company to help farmers, seed dealers, and company personnel determine the likely causes of abnormal corn plant appearance. While the primary target is the North American Region, the principles and many of the situations described in this publication extend to the regions of the world where corn is grown. Symptoms may be due to a single cause or the result of two or more interacting factors. Also, because of their different genetic backgrounds, different hybrids may not have identical symptoms in response to the same cause. Routine field examinations are crucial in spotting problems or potential problems. Some growers employ professional crop scouts rather than perform this function themselves. Weekly examinations are generally sufficient. Once a problem has been identified, its extent and severity must be determined to decide whether corrective action is necessary. Many pest management recommendations include threshold levels when control measures will return a profit. Consult Cooperative Extension Service and chemical company recommendations for control measures. 1 Corn Diagnostic Guide

5 1Diagnosing Field Problems in Corn EXAMINING A FIELD Carry the appropriate tools to help effectively scout or monitor fields. Some basic tools to have on hand include: Tape measure Knife Trowel (6 or 8 inch) Magnifying glass Clipboard and record keeping materials Small plastic bags Water source Paper towels Calculator Field marking flags A spade and a set of nesting pails may be useful if considerable digging is expected. Digital cameras can be helpful in getting a record of insects or disease symptoms. Be very careful when making area-to-area or field-to-field comparisons. Many factors can influence appearance, including: soil type, slope, and drainage; previous crops; fertility practices; seedbed preparation; date, depth, and rate of planting; pest control; seed lot; and hybrid. Look for positives, not just problems. Observe and note hybrid differences, as well as the effects of fertilizer and cultural practices. The following diagnostic key separates plant growth into three primary stages of crop development: Stage I Stage II Stage III Emergence to knee-high Knee-high to tasseling Tasseling to maturity In the field, onset of symptoms might occur earlier or later than indicated, and may be observed during more than one growth stage. TIPS FOR SHIPPING TO DIAGNOSTIC LABORATORIES Trained, experienced agronomists, crop protection, research, and sales personnel stand ready to assist you in diagnosing field problems. Local seed dealers are your first contact when questions arise. Positive diagnosis often requires identification or confirmation of causes by a diagnostic laboratory. Most states have some arrangement, usually through their Cooperative Extension Service office, for accessing expert diagnosis. Some of these are more formally conducted than others, and cost of the service varies. Also, there are a few private laboratories that are equipped to provide such services. Your local Cooperative Extension Service office can suggest companies and provide contact information. When preparing plant or soil samples for a diagnosis, follow these instructions offered by the laboratories. 2

6 Provide representative samples of each problem observed, as well as a healthy sample from unaffected plants or plant parts. If sending leaf tissue, place sections of leaves showing disease symptoms between pieces of dry paper toweling or notebook paper. If sending the whole plant, remove excess soil from the roots and wrap the roots in moist paper toweling. Put roots and towel in a plastic bag. Do not place the entire plant in a plastic bag. Wrap stem and leaves with paper, foil, or cardboard. Do not add water or crush specimens unnecessarily. Use a sturdy envelope or box for shipping. Avoid shipping at times that are likely to result in the parcel lying in a post office or freight depot over a weekend or holiday. Overnight delivery, early in the week, is strongly recommended. Information will need to be provided with the plant specimen. Often, laboratories have specific forms that are required when submitting a sample. Information requested on such forms often includes the following: Variety (hybrid number) of crop. Location where sample was taken (county, township, and town).. Date of planting, date problem was first observed, and date sample was collected. Indicate whether the problem is better or worse than when first observed. Visit a problem area twice, on dates about a week apart, to determine whether the problem is intensifying, spreading, or disappearing. Crop symptoms as observed in the field, such as plants were wilted or leaves appeared spotted. Describe the size of area affected: spots, strips, or the entire field. Percentage of plants affected. Soil type (clay, sand, muck, etc.). Topography around affected plants, such as high ground, low ground, or gently sloping. Fertility level (include a soil test report, if available) and the amount, kind, and timing of fertilizer application. Pesticides applied (fungicides, herbicides, insecticides, application rate, and date). Soil moisture situation at and since planting. If the field was irrigated, indicate the amount and dates of water application. Unusual recent air temperatures or humidity conditions. Previous cropping and tillage history. Types of weeds in the field. 3 Corn Diagnostic Guide

7 2Stage I: Scouting from Emergence to Knee-High Determine the extent and severity of any problem identified. Is the problem throughout the field or spotty and localized? Has emergence been completed or are there seeds sprouted, ready to emerge? Take accurate stand counts and determine percent of stand achieved. If replanting is necessary, take steps to prevent recurrence of the cause for poor emergence. Verify that the planter is operating properly and that fertilizer or pesticide issues have been corrected. number of plants established % stand achieved = X 100 number of seeds planted If the stand is uneven or if there are skips down the row, dig to find the planted seed and its distribution. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 1. No seed Planter Rodents or birds Improper adjustment; row unit drive not engaged; worn parts; clogged spout; empty box or tank; wrong plates, disks, or drum; excess or wrong seed treatment. Digging and partly-eaten kernels. 2. Normal seed appearance; not swelled Unfavorable soil conditions Poor seed-soil contact Cold, dry soil. Inadequate press wheel pressure; improper closing wheel adjustments; inadequate residue management; dry or cloddy soil. 3. Normal seed appearance; swelled but not sprouted Unfavorable soil conditions Fertilizer or pesticide injury Cold, wet soil. Phytotoxic pesticides or too much fertilizer too close to the seed. 4. Seed dead, rotted Seed rots or seedling blights These are accentuated when soil conditions are unfavorable for germination and seedling growth. Many species of fungi and/or bacteria may be involved. Fungicide seed treatment protects the seed, not the seedling. 4

8 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 4. Seed dead, rotted (cont.) Fertilizer injury Fertilizer salts, nitrogen, and potassium draw moisture and may leave seed in soil too dry to support growth. In-furrow applications are more likely to cause fertilizer injury than starter fertilizers placed at least 2 inches from the seed. Ammonia toxicity is caused when planting follows anhydrous or aqua ammonia application too closely or where application was too shallow. This can kill or stunt seedlings. Roots appear sheared off. Boron and some other micronutrients impair germination if they are too close to the seed. Insecticide injury Some soil-applied organophosphate insecticides can impair germination if placed in furrow with the seed. Check the label and apply only as directed. Dead seed planted Unfavorable soil conditions Cold, dry, wet, or crusted soil. 5. Seed hollowed out Insects Seed corn beetle, seed corn maggot, or wireworm (see pp ). 6. Sprout twisted or leaves expanded below ground Unfavorable soil conditions Crusted, cold, or cloddy soil. A cloddy surface can allow light to reach the sprout and trigger leafing too soon. In the case of crusting, rotary hoeing may be beneficial. Seed planted too deep Mechanical injury to seed in handling or planting Chemical injury Fertilizer (see p. 5); insecticides (see p. 5); or some herbicides such as acetanilides and dinitroanilines (see pp ). 7. Slow, uneven emergence Planter Seed injury due to improper operation or adjustment, including planting depth. Unfavorable soil conditions Cold, dry, wet, or crusted soil. In the case of crusting, rotary hoeing may be beneficial. Properly banded fertilizer at planting may help seedlings overcome unfavorable soil conditions. Seed planted too deep 5 Corn Diagnostic Guide

9 If plants are abnormal in appearance, try to identify one of the following specific symptoms. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 1. Seedlings pulled or dug up, seed eaten Bird or rodent damage Chemical repellents may help. 2. Slow, uneven plant growth Unfavorable growing conditions Cold, dry, wet, or compacted soil. Properly banded fertilizer at planting may help minimize the effects of some unfavorable growing conditions. Low fertility If a nutrient deficiency due to inadequate amounts in the soil, is confirmed, consider sidedressing or foliar application, depending on the nutrients involved (see pp ). Insects attacking roots Corn root aphid, corn rootworm, grape colaspis, webworm, white grub, or wireworm (see pp ). Nematodes attacking roots Requires microscopic analysis. Chemical injury Fertilizer (see p. 5); insecticides (see p. 5); herbicides such as Balance ; Command or Scepter carryover (see pp ); or liquid manure. Non-uniform planting depth Failure of secondary roots to develop (rootless corn syndrome) Dry, loose soil is not conducive to normal root development. This condition is accentuated by shallow planting and whipping by wind. Cultivation may help by throwing soil around the base of plants. 3. Discolored leaves Nutrient deficiency Unfavorable soil conditions Magnesium, nitrogen, phosphorus, or sulfur (see pp ). Nitrogen deficiency results in a yellow discoloration of leaves. Phosphorus deficiency results in a purpling of leaves due to the accumulation of anthocyanins. If nutrient deficiency is confirmed, consider sidedressing or foliar application, depending on the nutrients involved. Waterlogged, cold, or compacted soil. These conditions can also affect nutrient uptake and translocation. 6

10 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 3. Discolored leaves (cont.) Insects attacking roots Most observed leaf discoloration is due to nutrient deficiency induced by damaged or inadequate roots (see nutrient deficiency, above). Nematodes attacking roots Requires microscopic analysis. Chemical injury Fertilizer (see p. 5); insecticides (see p. 5); herbicides such as Balance ; Command or Scepter carryover (see pp ); liquid manure. Wind damage Abrasion by sand or soil particles. Difficulty establishing secondary roots. Frost or freeze Check growing point for damage. Seedlings often recover. Most pronounced in low-lying areas. Cold (not freezing) temperature stress Cool nights and warm days promote aboveground plant growth at the expense of root development. This leads to increased demand by the above ground tissues for more nutrients than the roots can deliver. The result can be short-term deficiency symptoms until the root system becomes more developed. Anhydrous burn Mechanical injury Hybrid differences Uneven Corn Plant Growth 7 Corn Diagnostic Guide

11 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 4. Leaves rolled or puckered, may be wilted Drought Insects attacking roots or stalks See Section 6, Insect Injury Symptoms (p ), plus chinch bug, cutworm, Japanese beetle, stink bug, or webworm. Nematodes attacking roots Requires microscopic analysis. Mechanical root pruning 5. Leaves rolled or twisted together ( onion leaf or buggy whip ) Herbicide injury Temperature variation Acetanilides, dinitroanilines, or phenoxys (see pp ). Alternating hot and cold weather, inducing very rapid growth spurts followed by little or no growth. Nutrient imbalance Boron toxicity or calcium deficiency (see p. 52). Hail damage Injury to the growing point at this growth stage can result in short-term wrapping of leaves. 6. Shredded leaves or eaten plants Wind damage Hail damage Insects Armyworm, common stalk borer, corn earworm, cutworm, European corn borer, grasshopper, slug, or webworm (see pp ). Livestock or wild animal grazing Look for tracks. 7. Leaves spotted, striped or dead Wind damage Low soil ph Abrasion by sand or soil particles. Beaded streaking of leaves, which turn reddishpurple and may die. Nutrient deficiency Boron, copper, magnesium, potassium, sulfur, or zinc (see pp ). Insects Flea beetle, leaf miner, or thrips (see pp ). Disease Anthracnose, bacterial wilt, eyespot, Goss s wilt, holcus spot, seedling blights (favored by cool, wet soil), virus or virus-like diseases (see pp ). Fertilizer or herbicide injury Anhydrous burn; spray drift; foliar-applied herbicides such as Buctril, Basagran, Gramoxone, or Blazer ; Classic, Scepter, or Reflex carryover; premix acentanilides postapplied (see pp ). Sunscald or cold Hybrid differences Cold nights followed by clear, bright, fast-warming days. 8

12 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 8. Rows of holes across leaves Insects Billbug, common stalk borer, corn borer, cutworm, or stink bug (see pp ). 9. Plants wilt and die suddenly Insects Wind damage Disease Herbicide injury Frost or freeze Lightning Anhydrous burn Flooded, water-logged soil Billbug, chinch bug, cutworm, stink bug, white grub, or wireworm (see pp ). Seedling blights, bacterial wilt, or Goss s wilt (see p. 26). Triazines or misapplications of glyphosate herbicides (see pp ). Check growing point for damage. Seedlings often recover. Kills everything, usually in circular area. Buggywhip Corn plant two days after frost 9 Corn Diagnostic Guide

13 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 10. Plants twisted or broken off 11. Inhibited root development or malformed roots Herbicide injury Insects Nematode injury Insects Fertilizer injury Herbicide injury Soil conditions Especially 2,4-D followed by wind (see pp ). Billbug, cutworm, lesser cornstalk borer, or stinkbug (see pp. 32, 36-37). Requires microscopic analysis. Corn rootworm, grape colaspis, white grub, or wireworm (see pp ). Phenoxys, Banvel, carryover dinitroanilines, and Scepter or Classic carryover (see pp ). Planting when soils are too wet can cause sidewall compaction that can arrest or severely restrict corn root development. Look for flatsided or abruptly arrested root systems. Wind Damage in Corn Field Hail Damage in Corn Field 10

14 3Stage II: Scouting from Knee-High to Tasseling This is the period of most rapid plant growth. Nutrient and moisture demands are high; deficiencies will likely reduce crop yield potential. Problems must be evaluated for economic damage potential before control decisions can be made. Observe differences due to hybrids and management practices. If plants are abnormal in appearance, try to identify one of the following specific symptoms. GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 1. Uneven height (tall plants, short plants) Emerged at different times Varied nutrient or moisture availability under drought conditions Uneven planting depth; uneven soil moisture. Low soil ph Herbicide drift Use of burndown products adjacent to cropped area. Nutrients concentrated at dry soil surface Nutrients are unavailable to the plant. Fallow (idle land) syndrome Phosphorus is unavailable to the plant (see p. 53). 2. Numerous tillers Growing point injury Favorable early-season growing conditions Low plant population Adjacent to open spaces (population gaps) in the row Hybrid differences Disease Mechanical or insect damage. Optimum moisture, high soil fertility, or both. Crazy top (see p. 27) 3. Discolored or dead leaves Nutrient deficiency Nitrogen, phosphorus, or potassium (see p. 53). 11 Corn Diagnostic Guide

15 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 3. Discolored or dead leaves (cont.) Fertilizer or herbicide injury Fertilizer or herbicide on foliage. This tends to be more pronounced at row ends or where overlap application was made. High temperatures Noted as scalding or bleaching of top leaves first. Sunscald or cold banding Cold nights followed by clear, bright, fastwarming days. Insects Chinch bug, spider mite, or root-attacking insects, which can reduce nutrient uptake (see pp ). Mechanical injury Frost or freeze Hybrid differences Barren stalk Purpling or reddening due to anthocyanin expression in response to accumulation of sugars that cannot translocate to the initiating ear. 4. Whorl leaves dead ( deadheart ) Disease Insects Bacterial stalk rot (see p. 28). Billbug, corn borer, or fall armyworm (see pp ). Chemical injury More common following a late over-the-top application of certain herbicides. Excessive heat Temperature exceeding 100 F, depending on moisture availability and genetics of the hybrid. 5. Leaves spotted or striped Low soil ph Beaded streaking of leaves, which turn reddishpurple and may die. Nutrient deficiency Boron, iron, magnesium, manganese, nitrogen, or potassium (see p ). Chemical injury Herbicides, additives reactions, or fertilizers; symptoms may be due to spray drift on foliage. Disease Bacterial leaf blight, eyespot, Goss s wilt, holcus spot, leaf blights, Physoderma brown spot, rust, sorghum downy mildew, virus or virus-like diseases (see pp ). Insects Cereal leaf beetle, corn rootworm beetle, corn blotch leaf miner, flea beetle, spider mite, or thrips; root-damaging insects induce nutrient deficiency or drought symptoms (see pp ). 12

16 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 5. Leaves spotted or striped (cont.) Wind damage Hail damage Abrasion by sand or soil particles. Genetic stripe Observed only on occasional plants. Sunscald or cold banding Cold nights followed by clear, bright, fastwarming days. 6. Leaves eaten or shredded Livestock or wild animals Insects Look for tracks. Armyworm, billbug, common stalk borer, corn borer, corn earworm, cutworm, grasshopper, Japanese beetle, leaf miner, slug, or wireworm (see pp ). Hail damage Determine if the growing point survived. If it did not, plant will not produce a tassel. Despite this, ear fertilization may still occur. Wind damage Disease Sorghum downy mildew (see p. 28) Plants stunted, leaves close together with mosaic mottle or streaks; leaves yellow or red Gray or black galls growing on plants Virus or virus-like diseases Corn lethal necrosis, corn stunt spiroplasma, maize chlorotic dwarf, or maize dwarf mosaic (see p. 27). Disease Common corn smut (see p. 26). 9. Stalks spindly, unthrifty, yellow Nutrient deficiency Excess soil moisture See pp Excessive plant population Root damage Insects, nematodes, chemical, or mechanical. Stalk damage Insects, disease, or mechanical. 10. Plants wilted or rolled Drought Root damage Stalk damage Insects, nematodes, chemical, or mechanical. Mechanical or insects; chinch bug, common stalk borer, or corn borer (see pp ). 11. Top leaves tightly rolled ( buggy whip or onion leafing ) Chemical injury Cold soil during seedling stage 2,4-D and other phenoxy herbicides; Banvel (see p. 41). 13 Corn Diagnostic Guide

17 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 11. Top leaves tightly rolled ( buggy whip or onion leafing ) (cont.) 12. Plants growing up in a curved gooseneck 13. Curled or stubby brace roots 14. Plants twisted or growing off at 90 angle 15. Plants leaning or broken off Temperature response or rapid growth syndrome Nutrition Mechanical injury Hybrid differences Recovery after root damage Recovery after early season environmental conditions Wind and wet soils Dry, hot soil surface Wind action Mechanical injury Wind damage Livestock or wild animals Root damage Alternating hot and cold periods, speeding and slowing plant growth. Boron toxicity or calcium deficiency (see p. 52). Insects (usually, but not limited to corn rootworm larvae), nematodes, herbicides (especially phenoxys), soil compaction, mechanical injury, or wind. Especially if soil is wet. Wind can cause greensnap at lower nodes of rapidly growing stalk. Look for tracks. Insects, nematodes, chemical, or mechanical. 16. Plants wilt and die suddenly Insects attacking stalk Chemical injury, especially if followed by wind Soil compaction Poor secondary root development in dry soil Disease Hail damage Lightning Drought Common stalk borer or corn borer (see p ). Especially Banvel or phenoxy herbicides (see pp ). Inadequate root development. Rootless corn syndrome due to weather and aggravated by shallow planting. Bacterial or Pythium stalk rots (see pp ). Kills everything, usually in a circular area. Field areas with low water-holding capacity are first to show stress. 14

18 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 16. Plants wilt and die suddenly (cont.) Disease Frost or freeze Herbicide injury Bacterial or Pythium stalk rots (see pp ). Misapplication of glyphosate, glufosinate, paraquat, or other non-selective herbicides (see pp. 41). 17. Tassel feeding (while in whorl) 18. Tassels proliferated into green cluster of fingerlike branches, excessive tillering, leaves often thick and straplike 19. Tassel proliferated, with stringy black vascular bundles present Insects Armyworm, corn leaf aphid, or western bean cutworm (see pp ). Disease Crazy top (see p. 27). Disease Head smut (see p. 26). Greensnap at lower nodes of rapidly growing corn stalk 15 Corn Diagnostic Guide

19 4Stage III: Scouting from Tasseling to Maturity This period, which commences with the critical pollination stage, includes grain fill, plant maturation, and death. It also includes the critical stage before harvest when standability and ear retention become concerns. Field observations are essential to detect yield-depressing factors during this period. If LEAVES are affected, try to identify one of the following specific symptoms GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 1. Eaten Livestock or wild animals Insects Look for tracks. Armyworm, corn borer, corn rootworm beetles, grasshopper, or Japanese beetle (see pp ). 2. Shredded Hail damage Wind damage Disease Sorghum downy mildew (see p. 28). 3. Dead frosted appearance Frost or freeze Drought Disease Insects Anthracnose, leaf blights, or stalk and root rots (see pp ; 28-29). Corn borer, corn leaf aphid, or spider mite (see pp ; 37). 4. Spotted or dead Disease Chemical injury 5. Discolored sheath Insects Disease Numerous leaf blights such as anthracnose, eyespot, Goss s wilt, gray leaf spot, Helminthosporium leaf spot, northern corn leaf blight, Phaeosphaeria leaf spot, Physoderma brown spot, rust, or southern leaf blight (see p. 26). Especially spray drift. Corn leaf aphid (see p. 31). Purple sheath spot caused by saprophytic organisms existing on pollen trapped between the leaf sheath and stalk. 16

20 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 6. Red or purple color Mechanical injury Barren plant Disease Insects Hybrid differences Anthocyanins develop in response to accumulation of sugars that cannot be translocated to filling grain. Corn rust, corn stunt spiroplasma, high plains virus, or maize chlorotic dwarf (see pp ). Corn borer (see p. 33). 7. White striping with white, downy growth on upper and lower leaf surfaces Disease Sorghum downy mildew (see p. 28). If SILKS are affected, try to identify one of the following specific symptoms GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 1. Eaten off Insects Armyworm, corn earworm, corn rootworm beetle, grasshopper, Japanese beetle, or western bean cutworm (see pp ). 2. None visible, or delayed several days after tasseling Heat Drought Nutrient deficiency or imbalance Especially nitrogen or phosphorus. Temperature variation Cold nights occurring just prior to silking. Insects Corn leaf aphid, fall armyworm, or spider mite (see pp ; 37). Excessive population for conditions 3. Trapped, balled within the husk Drought Nutrient deficiency or imbalance See pp Temperature variation Hybrid differences Cold nights during early silking. 4. Red or green color Hybrid differences Fresh silk color is genetically controlled. 17 Corn Diagnostic Guide

21 If TASSELS are affected, try to identify one of the following specific symptoms GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 1. Failed to emerge Drought or heat stress Nutrient deficiency Insects Especially boron (see p ). Aphid stress, corn earworm, or fall armyworm (see pp ). 2. A mass of leaves Disease Crazy top, head smut, or sorghum downy mildew (see pp ). 3. One or more small ears formed Genotype by environment interaction Most common on tillers. 4. Kernels develop in tassel Genotype by environment interaction Most common on tillers. 5. Broken off, stalk tunneled Insects Corn borer (see p. 33). If STALKS are affected, try to identify one of the following specific symptoms GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 1. Red or purple color Mechanical injury to plant Barren stalk Hybrid differences Insects Anthocyanins develop in response to accumulation of sugars that cannot be translocated to filling grain. When corn borers tunnels into a stalk, the area above the point of entry may turn red or purple. 2. Barren (no ear enlargement) Drought Heat 18

22 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 2. Barren (no ear enlargement) (cont.) Nutrient deficiency or imbalance Insects See pp Corn leaf aphid or spider mite (see pp. 31, 37). Silks eaten off prior to pollination Corn rootworm beetle and Japanese beetle (see pp ). Disease Head smut, mildews, virus or virus-like disease (see pp ). Excess population for conditions Tillers Seldom produce a fully-developed ear. Timing of silk or pollen shed disrupted Environmental stress. Mechanical injury 3. Broken below ear Wind Disease Nutrient imbalance Insects Weather stress 4. Broken above ear Wind Insects Mechanical injury Excessive population for conditions Delayed harvest Mechanical injury Delayed harvest Plants just prior to tasseling are especially vulnerable to greensnap by wind. Stalk and root rots (see pp ). Excess nitrogen, insufficient potassium (see p. 53). Southwestern corn borer (see p. 33). Drought, heat, or other conditions limiting photosynthesis. Machinery; livestock or wild animals. European corn borer or fall armyworm (see pp ). Machinery; livestock or wild animals. 5. Multiple ears at one node Hybrid differences Mechanical injury Cold temperature when ears formed Disease Mildews, virus or virus-like disease (see pp ). 19 Corn Diagnostic Guide

23 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 5. Multiple ears at one node (cont.) Insects Results from severe silk clipping by insects such as corn rootworm beetle and Japanese beetle (see pp. 34, 36). 6. Leaning, but not broken (root lodged) Wind and wet soil Poor root development Drought, fertilizer placement, soil compaction, low soil ph, or poorly drained soil. Insects Corn rootworm and other root feeders (see pp ). Nematode activity Disease Stalk and root rots (see pp ). Nutrient deficiency Especially potassium (see p. 53). Herbicide injury Especially 2,4-D and other phenoxys (see p. 41). Mechanical injury Machinery, livestock or wild animals Delayed harvest Hybrid differences 7. Premature death Disease Insects Frost or freeze Severe drought Lightning Leaf blights or stalk and root rots (see pp , 28-29). Corn borer (see p. 34). Kills everything, usually in a circular area. 8. Black mold Saprophytic Buildup on dead stalk tissue under warm, humid weather conditions. Timely harvest will deter. If EARS are affected, try to identify one of the following specific symptoms GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 1. Numerous ear shoots, leafy and barren Disease Crazy top, sorghum downy mildew, virus or virus-like diseases (see pp ). 20

24 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 2. Soft, glistening smut galls, black and powdery when mature Disease Common corn smut (see p. 26). 3. Poorly filled tips Hybrid differences Nutrient deficiency Interaction between population, hybrid, and environment Insects Disease Drought Unusually favorable growing conditions after pollination Suboptimum light, carbon dioxide, or temperature during kernel fill Especially nitrogen or potassium (see p. 53). Silks clipped before pollination. Corn borer, corn earworm, corn rootworm beetle, fall armyworm, or Japanese beetle (see pp ). Foliar disease that reduce photosynthesizing area (see pp ). Silks at ear tips were not present when pollen was available Cob elongates, but rarely fills up tip kernel. Reduced photosynthesis; tip kernels abort. 4. Small malformed, light weight Nutrient deficiency Drought or heat stress Especially nitrogen and phosphorus (see p. 48). Plant damage Mechanical or disease. Excessive population for conditions Second or third ear on stalk, or on a tiller Insect-caused plant stress Numerous ear-feeding insects (see pp ). Disease Leaf blights, stalk and ear rots, virus or virus-like diseases (see pp ). Soil compaction 5. Very short husk, remains tight at maturity; ear tip exposed Weather Usually observed after drought or heat has stopped husk growth, but later favorable conditions permit more normal ear size to develop. Hybrid differences 21 Corn Diagnostic Guide

25 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 6. Barren (little to no grain) Disease Head smut, virus or virus-like diseases (see pp ). Pollination failure Drought, heat, or other stress interfered with silk/pollen shed timing; insect (clipped silks or caused tassel to abort); chemical injury (especially by growth regulators like 2,4-D and Banvel ). Pesticide injury Following drift or misapplications. 7. Pinched ear syndrome Chilling injury during early ear development 8. Dropped Drought Nutrient deficiency Hybrid differences Insect damage to shank 9. Scattered kernel set Lack of adequate viable pollen when silks are receptive Silks eaten off before pollination Heat or drought at pollination Nutrient deficiency Herbicide injury Also called beer can ear. Ear is reduced to 3-4 inches. Well filled kernels are on bottom third of ear, cob tissue on middle third, and undeveloped tissue on top third. Weakened shank. Weakened shank. Ears usually drop free of husk. Husk usually remains on the dropped ear; European corn borer (see p. 33). Insect or animals. Phenoxy herbicides or too-late applications of glyphosate (see p. 41). 10. Kernel feeding Insects Birds, animals Corn borer, corn earworm, fall armyworm, picnic beetle, or western bean cutworm (see pp ). 11. Rotten (spots or entire ear) Disease Insects Birds Hail or other mechanical injury Ear and kernel rots or Helminthosporium leaf spot (see pp ; 25). Corn borer, corn earworm, fall armyworm, picnic beetle, or western bean cutworm (see pp ). Followed by weathering or disease. Followed by weathering or disease. Followed by weathering or disease. 22

26 GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 12. Kernels eaten from ear tips 13. Tunnels in ear, cob, shank, or stalk Birds Wild animals or rodents Insects Corn earworm, fall armyworm, picnic beetle, or western bean cutworm (see pp ). Insects Corn borer, corn earworm, or fall armyworm (see pp ). If KERNELS are affected, try to identify one of the following specific symptoms GENERAL SYMPTOM POSSIBLE CAUSE REMARKS 1. Broken seedcoat, popped appearance Genetic and environment interaction Popped kernels usually occur under high temperatures Horizontally cut or split seedcoat Pink or red streaking or lengthwise stripes, especially running over crown Silk-cut A genetic and environment interaction. Not visible until kernels are shelled from the cob. Kernel red streak Caused by toxin secreted during feeding of wheat curl mite. More pronounced toward ear tip. No detrimental effects are known. 4. Sprouted especially at base of ear High rainfall and warm temperature while ear remained erect on stalk Usually accompanied by mold. 5. White streaking, also known as starburst Disease Associated with Fusarium ear rot. Barren corn Pinched ear syndrome 23 Corn Diagnostic Guide

27 5 Disease Symptoms This section will help you identify the common corn diseases based on plant symptoms and conditions. It is not uncommon to have symptoms of several different diseases present at the same time. Symptoms of different diseases may appear similar, particularly during early stages of disease development. Laboratory culturing and microscopic examination may be required to make a positive identification. Growth stages during which symptoms generally appear are listed following the disease name and are described as follows: Stage I Stage II Stage III Emergence to knee-high Knee-high to tasseling Tasseling to maturity LEAF DISEASES Anthracnose (Stage I, III) Oval to spindle-shaped water-soaked lesions on youngest leaves turn tan to brown with yellow to reddish brown borders. Heavily infected leaves wither and die. Top-leaf die-back may occur 4 weeks to 6 weeks after pollination, leaving the lower stalk green. The organism thrives in warm, humid weather. The stalk-rot phase of the organism is of greater concern than the leaf blight phase. Bacterial Wilt and Bacterial Leaf Blight (Stewart s Wilt, Stewart s Disease) (Stage I, III) Young plants exhibit long, green-gray, water-soaked lesions with wavy margins, accompanied by stunting and wilting which lead to plant death. Cavities may form in stalk near the soil line. Bacterial masses ooze from cut end of infected stalks or leaves. The more common leaf blight phase appears after tasseling. Leaves are streaked with gray-green to yellowgreen lesions, each distinguished by the presence of a flea beetle feeding scar toward the base of the streak. Streaks are long and irregular, turning tan as the tissue dies. Flea beetles (small, oval, black insects) are the primary carrier. Incidence of the disease is relative to beetle population. Sweet corn tends to be more sensitive than field corn to this disease. Common Corn Rust (Stage II, III) Cinnamon-brown, powdery, circular-to-elongated pustules (blister-like growths) can occur on any aboveground plant tissue, but especially on both surfaces of the leaves. In contrast, pustules of southern corn rust occur primarily on the upper leaf surface. Pustules rupture leaf surface and rusty powder can be rubbed off with fingers. Pustules become dark brown to black late in the growing season. The organism thrives in moderate to cool temperatures and high humidity. 24

28 Eyespot (Stage II, III) Small (less than ¼ inch), circular, translucent lesions surrounded by a yellow to purple margin, give a halo effect. Lesions occur on leaves (most commonly as plants approach maturity), sheath and husk. The disease is favored by cool, moist weather. Goss s Wilt (Leaf Freckles and Wilt, Nebraska Bacterial Wilt and Leaf Freckles) (Stage I, III) Young plants wilt and die. Vascular bundles are discolored. More common later-season infections produce dull gray-green to orange lesions forming water-soaked streaks with irregular margins on leaves. Within developing lesions, small, irregular shaped watersoaked freckles appear. Bacterial droplets may ooze from the leaf surface early in the morning. Plant injury, such as from hail or wind damage, enhances infection. Gray Leaf Spot (Stage III) Gray to tan, rectangular lesions on leaf, sheath or husk tissue. Spots are opaque and long (up to 2 inches). Lower leaves are affected first, usually not until after silking. The organism thrives in extended periods of warm, overcast days and high humidity. It has become more prevalent with increased use of reduced tillage and continuous corn. Helminthosporium Leaf Spot (Northern Leaf Spot) (Stage III) Numerous races of the organism have been identified. Symptoms vary by race. Lesions tend to be oblong to blocky, and tan to brown in color. This leaf spot may also cause a black, charred-appearing ear rot. It prefers moderate temperatures and high humidity. Holcus Spot (Stage I) This organism causes small, circular to oblong, watersoaked lesions toward tips of lower leaves. Later, lesions become creamy white to tan, then light brown with reddish margins. Holcus spot is very similar in appearance to paraquat damage. Rainstorms accompanied by wind splash overwintering bacteria from the residue onto young plant leaves. This organism does not cause serious loss. Northern Corn Leaf Blight (Stage II, III) Long (up to 6 inches), elliptical, gray-green lesions that become tan-brown identify infections caused by this organism. Infection spreads up the plant starting on lower leaves. It is favored by high humidity and moderate temperatures. Numerous physiologic races have been described. Host specific races of the organism may also attack sorghum. 25 Corn Diagnostic Guide

29 Phaeosphaeria Leaf Spot (Stage III) Lesions are initially small, round to oval, pale green or yellow, and scattered on leaves of mature plants. Spots become light tan with reddish-brown margins and may coalesce into irregularly shaped lesions. This disease is most prevalent in areas of high rainfall and moderate temperatures. Physoderma Brown Spot (Stage I, III) Small yellow spots appear first at the base of the leaf. These spots become brown and combine to form chocolate-brown to reddish irregular blotches, sometimes as bands of infection across leaf blades. Sheath, husk, tassel, stalk, and leaves may exhibit symptoms late in the season. Infected stalks may break at a node. This organism is favored by warm, wet weather. Southern Corn Leaf Blight (Stage II, III) The most common race, named O, produces small, elongated (up to 1 inch long), parallel-sided lesions that are tan with brownish borders. This blight primarily attacks leaves. It is favored by high humidity and warm temperatures. Southern Corn Rust (Stage II, III) Small, circular, orange-tolight-red pustules (blister-like swelling) occur on leaves, especially the upper surface, and sheaths. Pustules rarely break the leaf surface. (In contrast, common corn rust pustules attack upper and lower leaf surfaces and readily break through the epidermis.) This organism is favored by warm, humid weather. SMUTTING DISEASES Common Corn Smut (Boil Smut, Blister Smut) (Stage I, II, III) Local infection of any plant part, even below the soil surface, occurs through plant wounds or thin-walled cells of actively growing tissue. Resulting galls are first silverwhite, then become gray to black powdery masses of smut spores that are released when the galls break open. Galls on leaves seldom develop beyond pea-size and tend to harden and dry without rupturing. High fertility and plant injury favor the disease. Common smut is not toxic to animals. Head Smut (Stage III) Seedling infection results in systemic development of the disease as the plant grows and develops. Tassels of affected plants may multiply as a mass of leaves or be replaced by a black, smutty mass which ruptures to release spores, leaving black, thread-like vascular strands. Ears may be completely replaced by similar, often triangular-shaped, smut masses. Occasionally, part of an ear escapes and produces a few kernels. Hot, dry soil at seedling stage favors infection. One type of head smut also attacks sorghum. 26

30 VIRUS AND VIRUS-LIKE DISEASES Corn Lethal Necrosis (CLN) (Stage II, III) Mosaic patterns appear on leaves and husk. Leaves die from the margins inward and plants may die prematurely. The organism generally starts at the tassel and works downward. Barrenness or sharply reduced grain production is common. CLN is caused by synergistic interaction when plants become infected by maize chlorotic mottle virus and either maize dwarf mosaic virus or wheat streak mosaic virus. Presently, the disease has been identified only in parts of Kansas and Nebraska. Corn Stunt Spiroplasma (CSS) (Stage II, III) Corn stunt is caused by a spiroplasma, not a virus. Margins of whorl leaf turn yellow followed by reddening of older leaves and yellow striping which runs the length of leaves. Plants are stunted, have multiple tillers and produce numerous small UGA ear shoots. Root systems are reduced. Corn stunt symptom development and epidemiological patterns are similar to those of virus diseases. High Plains Virus (HPV) (Stage I, II, III) Symptoms begin as small yellowish flecks which often appear as lines running parallel to leaf veins. Infected seedlings turn bright yellow and quickly die. Plants may be stunted, older leaves may become red and then necrotic, and ear and kernel size may be reduced. Considerable symptom variation exists among cultivars. It is spread by the wheat curl mite. Maize Chlorotic Dwarf Virus (MCDV) (Stage II, III) Younger leaves are yellow and finely striped. There is a general yellowing or reddening of leaves and plants are stunted. Most affected plants are barren. MCDV is spread by leafhoppers from infected Johnsongrass and other host species. MCDV is detected more frequently and is more damaging than maize dwarf mosaic. Maize Dwarf Mosaic Virus (MDMV) (Stage II, III) Mottling of younger leaves progresses into narrow, lightgreen to yellowish streaks along leaf veins. Leaves, sheaths and husks may show symptoms. Plants are stunted and ear size and seed set are reduced. Sometimes, multiple tillers or ear shoots develop. MDMV is most prevalent in areas where Johnsongrass grows and serves as a winter host for the virus. It is spread by aphids. FUNGAL SYSTEMIC DISEASES Crazy Top (Stage II, III) Plants have excessive tillering with rolling or twisting of newer leaves. The tassel becomes a mass of leaves; ears, if present, often do likewise. Leaves are usually narrow, thick and appear strap-like. Occasionally oversized plants will develop. Infection occurs when young plants are under flooded soil conditions; therefore, it is more frequent in low-lying areas. This is one of numerous downy mildews that attack corn. 27 Corn Diagnostic Guide

31 Sorghum Downy Mildew (Stage II, III) Leaves are yellow with white stripes; plants are stunted and ears and tassels multiply into leafy masses. Often the base of leaves are chlorotic and sharply contrast with healthy green leaf tips. White, downy growth appears on either or both leaf surfaces. Leaves often split or shred. This is primarily a disease of sorghum. STALK AND ROOT ROT DISEASES Stalk rots are diseases that are most commonly expressed as plants reach maturity. Stalk rot of corn tends to be a complex of several disease-causing fungi and sometimes bacteria; seldom will only one causal organism be isolated and identified. Plants with rotted stalks almost always have rotted roots, too. Usually, but not always, the same causal organisms are involved. Visual identification is very difficult. Typically, wilting is the first sign of stalk rot in a field. In a few days, leaves turn a frosted gray, ears droop and the outer rind of the lower stalk turns brown. Fields where stalk rot is developing should be harvested early to reduce grain losses. Anthracnose Stalk Rot (Stage II, III) Early infection may kill plants before pollination, but onset usually occurs just before plants mature. Sometimes a portion of the plant above the ear blanches and dies prematurely (top dieback). Usually, the entire plant is killed and several nodes are rotted. Late in the season, a shiny black discoloration develops in blotches or streaks on the stalk surface, especially on lower internodes. Internal stalk tissue may become black and soft, starting at the nodes. Lodging typically occurs higher on the stalk than with other stalk rots. The same organism causes leaf blighting earlier in the season. Bacterial Stalk Rots (Stage II, III) At least two organisms have been identified as causing bacterial stalk rot. With either, there is rapid development of a soft rot of stalks, accompanied by wilting and plant death. Stalks twist and collapse while still green, disintegrating into a soft mass often accompanied by a foul odor. Charcoal Rot (Stage III) Charcoal rot can be expected when the crop has grown under hot, dry conditions; it also attacks sorghum and soybeans. Lower internodes are affected, causing premature ripening, shredding, and crown disintegration. Vascular strands remain intact but are black or charred in appearance. Stalks become gray-black or charcoal color. Diplodia (Stenocarpella) Stalk Rot (Stage III) Lower internodes are straw-brown, spongy, and dry. Pith disintegrates, leaving vascular strands intact. White fungal growth may appear on the stalk surface. Minute, dark bodies embedded just under the stalk surface are difficult to remove. 28

32 Fusarium Stalk Rot (Stage III) Rotting of roots, crown, and lower internodes leads to premature ripening and stalk breakage. Split stalks may show whitish-pink to salmon color. Disintegration starts at the nodes. Healthy stalk, left. Diseased stalk, right. Gibberella Stalk Rot (Stage III) Affected plants wilt, leaves turn dull gray-green, and the lower stalk softens and becomes straw colored as plants die. Pith tissue disintegrates, leaving only vascular strands. The inside of a rotted stalk is pink to red. Small, dark bodies are superficial on the lower stalk surface and can be scraped off easily. Pythium Stalk Rot (Stage II, III) This organism attacks individual plants in localized areas, sometimes before flowering. It is usually confined to the first internode above the soil line, where rind and pith develop wet rot. Stalks twist and collapse. Hot, wet weather favors development of this stalk rot. Red Root Rot (Stage III) Red root rot often occurs in plants that are also stalk rotted. Seminal roots are most damaged and adventitious root damage increases with their age. The root color of affected plants ranges from pink to royal red, with higher soil temperatures resulting in darker pigmentation. The role of this root rot in stalk lodging is not fully understood. EAR AND KERNEL ROTS These rots can affect ears, kernels, or cobs, reducing test weight and grain quality. Some rots are responsible for development of mycotoxins that may contaminate grain. Positive identification is difficult. Rotting observed in the field is often due to a complex of causal organisms, not just one. Most ear rots are favored by late-season humidity. Infections are increased by ear damage by birds or insects and by stalk lodging that allows ears to come into contact with the soil. Aspergillus Ear and Kernel Rot (Stage III) Greenish or yellowish-tan discoloration occurs on and between kernels, especially near the ear tip. Symptoms are more prevalent if the husk does not cover the ear tip. The rot is favored by hot, dry weather. It may produce aflatoxins. Cladosporium Ear Rot (Stage III) Dark gray to greenish black fungal growth causes kernels to appear blotched or streaked. Initial discoloration appears where kernels are attached to the cob. Infection eventually progresses upwards and infected kernels can be seen scattered over the ear. If completely colonized, ears are dark and lightweight. This disease is often associated with damage due to insects, hail, or frost. 29 Corn Diagnostic Guide

33 Diplodia (Stenocarpella) Ear Rot (Stage III) Symptoms include bleached husks, white mold over kernels, and rotted ears with tightly adhering husks. Early infection (2 to 4 weeks after silking) is likely to lead to complete ear rotting. Later infections may result in partial rotting, usually beginning at the base. Since corn is the only known host, this disease is most severe when corn is planted following corn in reduced tillage situations. Fusarium Kernel or Ear Rot (Stage III) Scattered individual or groups of kernels show whitish-pink to lavender fungal growth. Infected kernels may also have a starburst pattern of white streaks on the cap of the kernel or along the base. Infections are more frequent on damaged ear tips, and are favored by dry weather. Fusarium rot may produce mycotoxins. Gibberella Ear Rot (Stage III) Symptoms include reddish kernel discoloration, usually beginning at the ear tip. Husks may rot and be cemented to the ear. The organism is favored by cool, humid weather, particularly 2 to 3 weeks after silking. It produces several mycotoxins. Penicillium Ear Rot (Stage III) Powdery green to bluegreen mold develops on and between kernels. Infection usually begins at the ear tips and primarily occurs on ears with mechanical or insect damage. Infected kernels may become bleached and streaked. Blue eye occurs when the embryo becomes discolored due to the presence of blue-green fungal spores and can occur if infected grain is stored at high moisture levels. Trichoderma Ear Rot (Stage III) Dark green fungal growth is found on and between kernels and husks, often covering the entire ear. The disease usually occurs on ears with mechanical or insect damage. Infected plants tend to be widely distributed within a field. Photo Citations: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org (viewed 9/22/10); Corn Stunt William M. Brown Jr., Bugwood.org (viewed 9//22/10) High Plains Virus Howard F. Schwartz, Colorado State University, Bugwood. org (viewed 9/22/10) Bacterial Stalk Rot Holcus spot and Pythium stalk rot photos courtesy of Don White at University of Illinois 30